1. Field of the Invention
The present invention relates to a polarizer for an electro-optically activated or activatable cell with a polarization foil which is placed between a layer causing the adhesion to the cell and a layer giving protection against external influences, and to a method of producing such a polarizer.
2. Description of the Prior Art
Polarizers of the above-mentioned type are required on each of the two sides of an electro-optically activated cell, such as e.g., a twisted nematic liquid crystal cell utilizing the Schadt-Helfrich effect. Layer-shaped plastics polarizers are used preferably for this purpose. These polarizers are substantially constructed of two parts namely a polarizing or optically active layer, customarily foil-shaped, and either a single support foil present on one side of the polarizing layer and performing a protective function, or two support foils provided on the two sides of the polarizing layer. The first mentioned polarizer is customarily designated a "simple" or "asymmetrical polarizer" and the second-mentioned a "symmetrical" or "sandwich polarizer".
In the customary commercial construction, simple polarizers exhibit an approximately 130 .mu.m thick cellulose acetobutyrate (CAB) support foil, and as a polarizing layer a stretched polyvinyl alcohol (PVA) foil, such as is produced e.g., by the Polaroid Corporation, Polarizer Department, 20 Ames Street, Cambridge, Mass., 02139, USA and, Foster Grant Corporation, Ziralo Division, Van Nuys, California, USA, or a stretched polyvinylbutyrate (PVB) foil such as is produced, e.g., by American Polarizers, 141 South Seventh Street, Reading, Penn. 19603, USA. The thickness of the polarizing layer is between 10 and 20 .mu.m, so that the thickness of the polarizer is approximately 140 .mu.m.
When such simple polarizers are used in liquid crystal cells, these polarizers are provided on the polarizer foil side (PVA or PVB side) with a layer of a self-adhesive agent, so as to achieve the adhesion to the cell as well as the protection of the polarizing layer.
Sandwich polarizers exhibit, in their customary commercial construction, a stretched PVA foil which is provided on both sides with approximately 75 .mu.m thick cellulose triacetate support foils. Such polarizers, produced e.g. by Sanritsu Electric Co., Ltd., 1-30-13 Narimasu, Itabashi-ku, Tokyo, Japan are therefore approximately 170 .mu.m thick. They are characterized by high mechanical stability (no distortion), under frequent temperature variations and in the presence of moisture, and also high chemical stability in heat and moisture (little bleaching). There are also sandwich polarizers in which the polarization foil is arranged between two approximately 50 .mu.m thick acryplastics support foils. Polarizers of this type are characterized by a thickness of only 120 .mu.m, but are extremely sensitive to mechanical influences due to the softness of the acrylplastics foils.
In addition to the simple and the sandwich polarizers, there is a further polarizer, known as a "transfer polarizer", which is produced e.g., by the above-mentioned American Polarizers. This so-called "transfer polarizer" is very similar to the simple polarizer, from which it differs however in that the polarizing layer adheres only lightly to the support foil. The polarizing layer of this transfer polarizer can be applied to a glass surface without an adhesive medium, and the support foil can then be removed. The transfer polarizer is characterized by its small thickness of only 20 .mu.m, but it is unsuitable for use in liquid crystal cells due to consideration of cost because of the complicated process of application to the glass surface, and is furthermore extremely susceptible to mechanical and chemical influences.